This invention relates to a friction type continuously variable transmission for continuously adjusting the rotational speed of a shaft for driving a rotary member that is rotated at high speed, such as an impeller of a centrifugal blower, a centrifugal compressor or a radial turbine to rotate the impeller-carrying output shaft at a constant speed irrespective of the rotational speed of the input shaft.
FIG. 4 shows a conventional friction type continuously variable transmission. It has an input shaft 2 and an output shaft 3 which are rotatably mounted in a housing 1 on both sides thereof so as to be coaxial with each other. A plurality of double cones 4 are mounted in the housing 1 around the output shaft 3 so as to be rotatable about their own axes but not about the axis of the output shaft 3. They are rotatably supported on support shafts 6 of a carrier 5 that is movable axially of the output shaft 3.
The input shaft 2 has at one end thereof an input ring 7 kept in contact with one conical surface 4a of each double cone 4. The output shaft 3 is provided at its free end with a drive cone 8 that is kept in contact with the other conical surface 4b of each double cone 4.
The conical surfaces 4a, 4b of the double cones 4, the input ring 7 and the drive cone 8 are arranged such that the reaction force to the contact pressure between the input ring 7 and the double cones 4 that acts on the frictional contact surfaces of the double cones 4 due to the biasing force of springs 10 acts as axial forces on the input shaft 2 and the output shaft 3 to pull them away from each other.
The rotation of the input shaft 2 is transmitted to the double cones 4 through the input ring 7. The rotation of the double cones 4 is then transmitted to the output shaft 3 through the drive cone 8. An impeller or the like mounted on the output shaft 3 is thus rotated. The rotational speed of the output shaft 3 is variable by moving the double cones 4 axially of the output shaft 3 by means of a drive means 9 drivingly coupled to the carrier 5.
The double cones of this friction type continuously variable transmission are shaped so that their vertical section including the rotation axis is substantially symmetrical with respect to their maximum diameter where the peripheral speed is maximum. If this transmission is used as a step-up gear, its speed increasing ratio n is given by the following equation. ##EQU1## where d and c (see FIG. 4) are the radii of rotation of the input ring 7 and the double cones 4, respectively, at the contact portions between the input ring 7 and the double cones 4; and b and a are the radii of rotation of the double cones 4 and the drive cone 8, respectively, at the contact portions between the double cones 4 and the drive cone 8.
In order to increase the speed increasing ratio n, b has to be increased and/or c has to be reduced. Namely, if the section of the double cones 4 including their rotation axis is shaped substantially symmetrical with respect to their maximum diameter, the higher the speed increasing ratio n is set, the smaller the effective usable conical surface of the double cones 4 will be. Thus, in order to increase the speed increasing ratio, the dimension b has to be increased by using larger cones 4.
The speed increasing ratio attainable with this type of transmission (if used as a step-up gear) is typically about 7-26. But this figure varies from one arrangement to another. There is known a friction type continuously variable transmission which can change this ratio within a wider range.
When using such a friction type continuously variable transmission to drive a rotary member which are rotated at high speed, such as the impeller of a centrifugal compressor, it is desirable that it has a wide range of speed increasing ratio. But this often leads to a reduction in power transmission efficiency, which is not desirable for high-speed rotation application. The friction type continuously variable transmission shown in FIG. 4 is adapted for high-speed rotation but has a shortcoming in that its speed increasing ratio is relatively low.
If the transmission shown in FIG. 4 is used for a supercharger of an automobile, the rotational speed of the impeller will be kept low while the engine speed is low because of relatively low speed increasing ratio. Since the rotational speed of the impeller is low, it is impossible to sufficiently increase the pressure ratio. Namely, in order for the supercharger to be capable of producing enough compression work even at a low input rotational speed, it is imperative that the transmission have a sufficiently high speed increasing ratio.
Also, when using such a friction type continuously variable transmission, it is necessary to produce a high contact pressure at the contact surfaces between the rotary members for efficient torque transmission. Thus, it is necessary to provide such a transmission with torque-sensing load mechanism such as torque cams. A gear train also have to be coupled to the transmission. The entire mechanism is thus bulky and difficult to assemble.
In such an arrangement as shown in FIG. 4 in which the double cones have a symmetrical shape with respect to the maximum diameter, the double cones have to be large in size for large speed increasing ratio. This leads to increase in the size of the transmission itself.
Another problem of such a friction type continuously variable transmission is that if used to drive a rotary member that rotates at high speed, such as the impeller of a centrifugal compressor, the output shaft 3, which also rotates at high speed, tends to induce a large amplitude of vibration even with the slightest weight unbalance. Thus, it is necessary to perform balancing the output shaft 3 with high accuracy. In order to achieve better balancing of a long output shaft, mass adjustment is required in at least two axially separated correction planes of the output shaft. If the impeller is mounted on the left end of the output shaft 3, balancing at this end is done by removing unbalance weight of the impeller or the impeller mounting portion of the shaft. At the other end to which the drive cone 8 is mounted, balancing has to be done by removing unbalance weight of the drive cone itself. But changing the shape of the drive cone for balancing is not desirable because it has to be brought into contact with the double cones with high accuracy.
In FIG. 4, the diameter of the drive cone 8 at its extreme left end is sufficiently large compared with the diameter of the output shaft 3, so that the double cones 4 will never come into contact with the output shaft 3. But if it is desired to increase the speed ratio, the axial length of the conical surfaces 4b of the double cones 4 has to be increased. This increases the possibility of the double cones coming into contact with the output shaft while in operation.
An object of this invention is to provide a friction type continuously variable transmission whose speed increasing ratio can be set high, without increasing the size of double cones even while the input rotational speed is low, and which is compact and easy to assemble.